A polyamide fiber is excellent in toughness, adhesiveness, fatigue resistance and the like, and therefore is being widely used, for example, in a tire cord, a rubber-reinforcing cord for conveyer belt, power transmission belt, rubber hose and the like, a safety belt, a tent, a braid, a sewing thread and an airbag. Reducing the amount of a material such as fiber without impairing the function is required of these industrial materials and products.
Among others, a light-weight airbag mounted in a vehicle is necessary from the standpoint of improving fuel efficiency of a vehicle or compact cabin space. On the other hand, burst resistance sufficient to cause no rupture during high-speed and high-pressure deployment is also demanded to ensure safety in a car accident.
In recent years, a curtain airbag, side airbag, knee airbag, rear airbag and the like are being put into practice, in addition to a driver's seat airbag and a passenger seat airbag, and the required properties vary according to the storage portion or capacity. For example, in the case of a driver's seat airbag, an airbag fabric foldable into a compact configuration is demanded so as to ensure visibility in front of the vehicle or facilitate viewing of gauges. The curtain airbag needs to cover the entire side surface of a vehicle, and therefore has a large feature or a complicated shape and in addition, considering the sideway rolling of the vehicle after a side collision, the deployed bag is required to maintain an inflated state for a certain amount of time. Also, in the case of a side airbag, a knee airbag and the like, since the storage site and the passenger are close to each other and the bag actuation distance is restricted, the airbag must be deployed in a shorter time. Accordingly, it is more strongly demanded to enable high-speed deployment and enhance/maintaining burst resistance while realizing weight reduction and compactness.
Patent Documents 1 and 2 disclose a technique for obtaining a polyhexamethylene adipamide fiber having certain fiber structure properties, where at least 95 mol % is composed of a hexamethylene adipamide unit and the sulfuric acid relative viscosity is 3.0 or more. This fiber has a strength of 11.0 g/d or more, an elongation of 16% or more and a shrinkage in boiling water of 4% or less and is a so-called high-strength nylon 66 fiber, and such a nylon 66 fiber may be excellent in the mechanical properties but still has a problem with the yield in the weaving process or with the burst resistance in use as an airbag.
Patent Document 3 discloses a technique for obtaining a side airbag fabric having excellent performance in terms of storability, low impact and high-speed deployment. However, the fiber used in this technique has absolutely low fineness and, despite excellent performance in terms of storability, low impact and high-speed deployment, the mechanical strength is lacking. In addition, because of a polyester, the heat capacity is small as compared to a polyamide, causing a problem in airbag deployment where a deployment gas reaches a high temperature.